Tool of press forming

ABSTRACT

A press forming tool includes: a punch; and a die, and press forms a press forming product including a punch bottom and a shallow concave portion or a low convex portion in the punch bottom. The punch includes a punch bottom forming surface, a concave shape forming portion or a convex shape forming portion, and a low Young&#39;s modulus member installation region formed under the punch bottom forming surface and accommodating a low Young&#39;s modulus member having a lower Young&#39;s modulus than that of the punch bottom forming surface. The low Young&#39;s modulus member installation region includes an area directly under the concave shape forming portion or the convex shape forming portion of the punch and has a projected area in a press forming direction that is 110% or greater as large as a projected area of the concave shape forming portion or the convex shape forming portion.

FIELD

The present invention relates to a tool of press forming, particularly,relates to a tool for press forming a blank into a press forming productby pressing that has a punch bottom and a shallow concave portion or alow convex portion in the punch bottom. In the present application, theblank is a metal sheet made of a variety of metals, such as a hot rolledsteel sheet, a cold rolled steel sheet, a surface treated steel sheetmade by subjecting a steel sheet to surface treatment, including, forexample, electro galvanizing, hot-dip galvanizing, and organic filmtreatment, a stainless steel sheet, an aluminum sheet, and a magnesiumsheet. In the present application, the punch bottom is the bottom of apress forming product formed with a punch and a die. Specifically, thepunch bottom is formed with the punch pushed into the die. Furthermore,in the present application, the terms indicative of directions such as“directly under”, “under”, and “top” are used to refer to the verticaldirection in which the punch and the die are placed relative to oneanother in the direction of press forming. For example, if the die islocated above the punch, the terms “directly under” and “under” of thepunch indicate the direction opposite the die in the press formingdirection, whereas “top” of the punch indicates the direction of the diein the press forming direction.

BACKGROUND

Application of a high-tensile steel sheet to the automotive body hasbeen extensively increased in response to the request of fuel efficiencyand collision safety along with the weight reduction of the automotivebody. However, in manufacturing parts of an automotive body from ahigh-tensile steel sheet by press forming, formability and accuracy ofdimensions are likely to be reduced because of the low ductility of thehigh-tensile steel sheet and its large material strength, which has beenan issue to be solved. In addition, an increase in the forming loadapplied to form the parts requires changes in the press line andbreaking down of parts. These are problems to overcome when usinghigh-tensile steel sheets.

To overcome such issues, various technologies have been developed inorder to reduce the forming load applied during press forming on ahigh-tensile steel sheet. For example, Patent Literature 1 describes amethod of forming a metal sheet with a die and a punch. The methodreduces the stress of the metal sheet by stopping the punch during thecourse of operation starting with the punch contacting the metal sheetand ending with the punch reaching the stroke end, and resuming theforming operation of the metal sheet after a predetermined time elapsed,thereby reducing the forming load. Patent Literature 2 describes amethod allowing a reduction in the forming load applied during the pressforming process on a blank material with a die and a punch. In themethod, the blank material is sequentially formed by a predeterminedsection with several parts of the punch separately operating.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-open No.2011-245525

Patent Literature 2: Japanese Patent Application Laid-open No.2010-207907

SUMMARY Technical Problem

However, the method described in Patent Literature 1 needs a servo pressto stop the punch during the forming process, leading to a restrictionof the facility. With the method of Patent Literature 2, the complex diestructure increases the production cost. Furthermore, the method maycause fracture and wrinkles in the high-tensile steel sheet having lowformability, during press forming.

For example, as illustrated in FIG. 2, when forming a press formingproduct 11 having a concave portion 15 concaved in a punch bottom 13 andhaving a depth close to the plate thickness in the press formingdirection, the forming load is markedly increased around the bottom deadcenter, as is common with operation of making a plate thinner bypressing. In such a forming process, reducing the forming load has beenan important issue. However, the methods of Patent Literatures 1 and 2are not intended to produce products like the press forming product 11,and therefore it has been difficult to sufficiently reduce the formingload applied in forming a concave or convex shape the depth of which isclose to the plate thickness. Therefore, technologies have been demandedto achieve a reduction in the forming load during formation of the pressforming product 11 having a shallow concave portion 15 (or a convexportion in a low convex shape in the press forming direction) in thepunch bottom 13.

From the above viewpoints, the present invention aims to provide a pressforming tool that allows a reduction in the forming load applied duringpress forming of a press forming product having a concave portion or aconvex portion concaved or convexed in the press forming direction inthe punch bottom, while eliminating the necessity of using a complexmechanism or structure.

Solution to Problem

A press forming tool according to the present invention includes: apunch; and a die, wherein the press forming tool is configured to pressform a press forming product including a punch bottom and a shallowconcave portion or a low convex portion in the punch bottom, the punchincludes a punch bottom forming surface to form the punch bottom, aconcave shape forming portion to form the concave portion or a convexshape forming portion to form the convex portion in the punch bottomforming surface, and a low Young's modulus member installation regionformed under the punch bottom forming surface and accommodating a lowYoung's modulus member having a lower Young's modulus than that of thepunch bottom forming surface, and the low Young's modulus memberinstallation region includes an area directly under the concave shapeforming portion or the convex shape forming portion of the punch and hasa projected area in a press forming direction that is 110% or greater aslarge as a projected area of the concave shape forming portion or theconvex shape forming portion. Advantageous Effects of Invention

The present invention relates to production of a press forming productby press forming that has a punch bottom and a shallow concave portionor a low convex portion in the punch bottom. A punch includes a lowYoung's modulus member installation region in which a low Young'smodulus member having a low Young's modulus is installed, under a punchbottom forming surface of the punch. During the process of pressforming, the punch bottom forming surface is flexed to the low Young'smodulus member installation region, which facilitates closeness betweena concave shape forming portion or a convex shape forming portion formedin the punch bottom forming surface and a blank. This configuration iseffective in minimizing an increase in the forming load applied aroundthe bottom dead center.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing illustrating the configuration of a press formingtool according to an embodiment of the present invention.

FIG. 2 is a drawing illustrating an example press forming product havinga concave portion concaved in the press forming direction in the punchbottom.

FIG. 3 is a drawing illustrating the configuration of a conventionalpress forming tool.

FIG. 4 is a schematic drawing illustrating deformation caused in theconventional press forming tool around the bottom dead center.

FIG. 5 is a schematic drawing illustrating deformation and flectioncaused in a press forming tool according to the embodiment of thepresent invention, around the bottom dead center.

FIG. 6 is a drawing illustrating the configuration of another aspect ofthe press forming tool according to the embodiment of the presentinvention.

FIG. 7 is a drawing indicating results of press forming analysesanalyzing the amount of deformation caused in a press forming toolaccording to a conventional example of the examples, around the bottomdead center.

FIG. 8 is a drawing illustrating results of press forming analysesanalyzing the amount of deformation caused in a press forming toolaccording to the invention example of the examples, around the bottomdead center.

FIG. 9 are graphs indicating the relation between the forming stroke andthe forming load in the process of press forming, analyzed by the pressforming analyses in the examples (the graph (a) indicates thefluctuations from start of forming until the bottom dead center, (b) isan enlargement view around the bottom dead center).

FIG. 10 is a graph indicating the results of forming load measured atthe bottom dead center in the conventional example and the example ofthe invention, of the examples.

FIG. 11 is a graph indicating the relation between the ratio of theprojected area A1 of the low Young's modulus member installation regionto the projected area A2 of the concave shape forming portion in thepress forming direction and the forming load, in the examples.

DESCRIPTION OF EMBODIMENTS

As illustrated in FIG. 1 as an example, a press forming tool 1 accordingto an embodiment of the present invention includes a punch 3, a die 5,and a blank holder 7. As exemplarily illustrated in FIG. 2, the pressforming tool 1 is used for press forming to press form a press formingproduct 11 having a punch bottom 13 and a concave portion 15 that isconcaved in the punch bottom 13 and is shallow. The shallow of theconcave portion 15 means that at least the depth is smaller than thepress forming height of the press forming product 11. For example, thedepth of the concave portion 15 in the press forming direction is 10 mmor smaller and is close to the plate thickness. Each component of thepress forming tool 1 according to the embodiment will now be described.In the following description, components having the same function willbe indicated by the same reference numeral, and description thereof willbe omitted.

The punch 3 includes a punch bottom forming surface 3 a to form thepunch bottom 13 of the press forming product 11, a concave shape formingportion 3 b to form the concave portion 15 in the punch bottom 13, and alow Young's modulus member installation region 3 d formed under thepunch bottom forming surface 3 a (opposite the die 5 in the pressforming direction) inside the punch 3 and accommodating a low Young'smodulus member 3 c having a lower Young's modulus than that of the punchbottom forming surface 3 a.

The low Young's modulus member 3 c is made of a material of whichYoung's modulus is lower than a casting and a steel material usuallyused for the punch 3 and the die 5. Examples of the material may includeresins, such as a rubber material and a urethane material, and metals,such as aluminum and magnesium.

The low Young's modulus member installation region 3 d includes the areadirectly under the concave shape forming portion 3 b, and has aprojected area A1 in the press forming direction that is 110% or greateras large as a projected area A2 of the concave shape forming portion 3b.

In the press forming tool 1 illustrated in FIG. 1, the low Young'smodulus member 3 c is installed so as to occupy the entire low Young'smodulus member installation region 3 d. The low Young's modulus member 3c is installed to the low Young's modulus member installation region 3 dby, for example, fittingly inserting the low Young's modulus member 3 cinto the punch 3, tightening with bolts, and fixing with adhesive.

The die 5 is placed facing the punch 3 with a blank 9 interposedtherebetween. The die 5 is designed to relatively move toward the punch3 in order to form the press forming product 11 in coordination with thepunch 3. The die 5 includes a punch bottom forming surface 5 a to formthe punch bottom 13 of the press forming product 11 and a concave shapeforming portion 5 b to form the concave portion 15 in the punch bottom13.

The blank holder 7 is disposed opposite the die 5, on the same side asthe punch 3 relative to the blank 9. The blank holder 7 is designed torelatively move toward the punch 3 while holding the ends of the blank 9with the die 5.

The operation of the press forming tool 1 in the above configurationaccording to the embodiment to produce the press forming product 11 bypress forming, and the effects of the operation will now be described incomparison to the case of using a conventional press forming tool 21illustrated in FIG. 3.

When the press forming product 11 is produced by press forming using theconventional press forming tool 21 including a punch 23, the die 5, andthe blank holder 7 as illustrated in FIG. 3, the blank 9 is placedbetween the punch 23 and the die 5 with the ends of the blank 9 pinchedby the die 5 and the blank holder 7. With the blank 9 pinched by the die5 and the blank holder 7, the die 5 and the blank holder 7 arerelatively moved toward the punch 23, thereby forming the punch bottom13. When the press forming product 11 is bottomed around the bottom deadcenter, the concave shape forming portion 5 b of the die 5 pushes theblank 9 into a concave shape forming portion 23 b of the punch 23,thereby forming the concave portion 15 in the punch bottom 13.

At the same time, around the bottom dead center, the surfaces of thepunch 23 and the die 5 are elastically deformed and pressed down in thedirection indicated by the arrows of FIG. 4, as illustrated in FIG. 4.The broken lines in FIG. 4 indicate the surface conditions of the punch23 and the die 5 before deformation. The areas hatched by the diagonallines are areas where the punch 23 and the die 5 are pressed down aroundthe bottom dead center.

However, since the punch 23 and the die 5 are not easily elasticallydeformed, the amount of deformation is small in each of them. In orderto form the concave portion 15 with the blank 9 sufficiently fitting theconcave shape forming portion 23 b of the punch 23, the blank 9 needs tobe pressed toward the punch 23 by applying large forming load, such thatthe plate thickness is made thinner by being pressed against the die 5,around the bottom dead center.

On the other hand, when the press forming tool 1 having the punch 3 andthe die 5 according to the embodiment produces the press forming product11 by press forming, as illustrated in FIG. 5, the surfaces of the punch3 and the die 5 are pressed down around the bottom dead center. Thebroken lines in FIG. 5 represent the surface conditions of the punch 3and the die 5 before deformation.

Specifically, as with the conventional press forming tool 21, thesurface of the die 5 is elastically deformed and pressed down around thebottom dead center (areas in the die 5 hatched with diagonal lines inFIG. 5). Since the low Young's modulus member installation region 3 d isarranged in the region including the area directly under the concaveshape forming portion 3 b, under the punch bottom forming surface 3 a,the punch bottom forming surface 3 a around the bottom dead center isflexed to the low Young's modulus member installation region 3 d whileinducing deformation of the surface of the punch 3 (the hatched areas inthe punch 3 in FIG. 5).

Because the amount of deformation of the punch 3 caused by the flectionof the punch bottom forming surface 3 a is larger than the amount ofelastic deformation of the punch 23 of the conventional press formingtool 21, closeness of the concave shape forming portion 3 b of the punch3 and the blank 9 is easily achieved. Consequently, the press formingtool 1 according to the embodiment can produce the press forming product11 having the concave portion 15 formed by press forming whileminimizing an increase in the forming load around the bottom deadcenter.

The present invention can further effectively minimize an increase inthe forming load around the bottom dead center, when the blank 9 to bepressed is a high-tensile steel sheet of 590 MPa or greater in the MPaclass.

The press forming tool 1 according to the present invention accommodatesthe low Young's modulus member 3 c inside the punch 3. This structurecan reduce the forming load with the punch 3 flexed during the formingprocess while ensuring the structural strength, even when the structureof the tool does not allow the punch 3 to be hollow (for example, whenthe press forming tool 1 is too small and thus does not allow the punch3 to be hollow to secure sufficient strength).

As described earlier, in the present invention, the projected area ofthe low Young's modulus member installation region 3 d of the punch 3 inthe press forming direction is 110% or greater as large as the projectedarea of the concave shape forming portion 3 b. This structure allows thepunch bottom forming surface 3 a to be flexed during the press formingprocess. The thickness of the punch bottom forming surface 3 a in thepress forming direction may be appropriately set in the range allowingthe flection of the punch bottom forming surface 3 a. The projected areaof the concave shape forming portion 3 b is the area projected from thebottom of the concave shape forming portion 3 b of the punch bottomforming surface 3 a.

In the above description, the punch 3 has the low Young's modulus member3 c installed in the low Young's modulus member installation region 3 din a fashion that entirely occupies the region. In another configurationof the present invention, as illustrated in FIG. 6, a press forming tool31 may have low Young's modulus members 33 c partially installed in alow Young's modulus member installation region 33 d of a punch 33.

With regard to the location and the area where the low Young's modulusmembers 33 c are installed, no restriction is provided if: the lowYoung's modulus members 33 c have the Young's modulus lower than that ofa punch bottom forming surface 33 a of the punch 33, the low Young'smodulus member installation region 33 d includes the area directly undera concave shape forming portion 33 b, and the projected area A1 in thepress forming direction is 110% or greater as large as the projectedarea A2 of the concave shape forming portion 33 b.

The manner of formation of the press forming product 11 having theconcave portion 15 concaved in the punch bottom 13 in the press formingdirection has been described above. Alternatively, the press formingtools 1 and 31 according to the present invention may be designed topress form a press forming product (not illustrated) that has a convexportion that is convexed in the punch bottom in the press formingdirection and is low.

The low convex portion means that at least the height of the convexportion is smaller than the forming height of the press forming product.The present invention is preferably applicable to such a press formingproduct that has the convex portion whose height is 10 mm or smaller andnearly equal to the plate thickness.

As described above, such a press forming product having a convex portionin the punch bottom can be produced by press forming using the pressforming tool according to the present invention. In the process offorming the convex portion around the bottom dead center, the die ispushed toward the punch, causing the punch bottom forming surface of thepunch to be flexed to the low Young's modulus member installation regionlocated below. This configuration can easily achieve closeness betweenthe blank and the convex shape forming portion of the punch. In thismanner, the convex portion is formed while minimizing an increase in thepress forming load applied around the bottom dead center.

The press forming tool 1 according to the embodiment has the die 5 abovethe punch 3 as illustrated in FIG. 1, with the direction in which thedie 5 descends to the punch 3 defined as the press forming direction. Inthe present invention, the press forming direction may be the directionin which the punch 3 is lifted to the die 5. Furthermore, in the pressforming tool according to the present invention, the die may be placedunder the punch with the punch configured to descend to the die or thedie configured to be lifted to the punch (not illustrated). The presentinvention is not intended to limit the position of the punch relative tothe die or the press forming direction.

In the above description, the press forming tool 1 illustrated in FIG. 1includes the blank holder 7 and produces the press forming product 11 bydraw forming with both ends of the blank 9 pinched with the die 5 andthe blank holder 7. The present invention may be applied to a pressforming tool having no blank holder and configured to produce the pressforming product 11 by crash forming with the punch 3 and the die 5.

EXAMPLES

Several experiments were conducted to validate the effects of the pressforming tool according to the present invention, and the detail thereofwill now be described.

In the experiments, the press forming process to produce the pressforming product 11 of FIG. 2 using the press forming tool 1 according tothe present invention illustrated in FIG. 1 was analyzed to evaluatedeformation of the punch 3 and the forming load applied during theprocess of press forming.

In the analyses of press forming, a high-tensile steel sheet having 1180MPa grade tensile strength and having a plate thickness of 1.2 mm wasused as the blank 9. The press forming product 11 is in a hat crosssection shape having the punch bottom 13 and the concave portion 15concaved in the punch bottom 13 in the press forming direction, asillustrated in FIG. 2. The dimension of each part is indicated in FIG.2. The concave portion 15 to be formed in the punch bottom 13 has adepth of 3 mm, which is smaller than the forming height (20 mm) of thepress forming product 11 and close to the plate thickness.

In the experiments, the press forming product 11 was produced using thepress forming tool 1 including the punch 3, the die 5, and the blankholder 7 as illustrated in FIG. 1. The analyses of the press formingprocess are described as an example of the invention. In the example ofthe invention, the punch 3 of the press forming tool 1 includes thepunch bottom forming surface 3 a, the concave shape forming portion 3 b,and the low Young's modulus member installation region 3 d accommodatingthe low Young's modulus member 3 c having Young's modulus lower thanthat of the punch bottom forming surface 3 a, under the punch bottomforming surface.

In the analyses of press forming, the punch 3 made of FC250 (gray castiron) having a Young's modulus of 115 GPa was used. The Young's modulusof the low Young's modulus member 3 c was set at 0.1 GPa that was lowerthan the Young's modulus of the punch bottom forming surface 3 a of thepunch 3. The low Young's modulus member installation region 3 d includesthe area directly under the concave shape forming portion 3 b, and hasthe projected area A1 in the press forming direction 110% as large asthe projected area A2 of the concave shape forming portion 3 b.

As a comparison, the press forming product 11 was produced using theconventional press forming tool 21 including the punch 23, the die 5,and the blank holder 7 illustrated in FIG. 3. The analyses of the pressforming process are described as a conventional example.

FIG. 7 and FIG. 8 illustrate the results of deformation of the punch 3,the punch 23, and the die 5 in the press forming direction measuredaround the bottom dead center in the conventional example and theexample of the invention. In the conventional example illustrated inFIG. 7, the concave shape forming portion 23 b of the punch 23 and theconcave shape forming portion 5 b of the die 5 were deformed in thepress forming direction at areas close to the edges. The largest amountof deformation was 0.02 mm. In the example of the invention illustratedin FIG. 8, deformation in the press forming direction was observed inextensive areas under the concave shape forming portion 3 b of the punch3 and above the concave shape forming portion 5 b of the die 5. Thelargest amount of deformation was 0.05 mm, which was a significantincrease from the conventional example.

FIG. 9 are graphs indicating the relation between the forming stroke andthe forming load in the press forming process. As is obvious from FIG.9, in the conventional example and the example of the invention, theforming load gradually increases after start of forming (the formingstroke 0 mm), and sharply increases around the bottom dead center (theforming stroke about 21 to 23 mm). As illustrated in FIGS. 9(a) and9(b), the forming load applied from the start of forming until aroundthe bottom dead center (the forming stroke 22.7 mm) was almost the samebetween the conventional example and the example of the invention. Onthe other hand, as illustrated in FIG. 9(b), the forming load around thebottom dead center was small in the example of the invention, incomparison to the conventional example.

FIG. 10 is a graph indicating the forming load of the conventionalexample and the example of the invention around the bottom dead center(forming stroke 23 mm). As indicated in FIG. 10, the forming load of theexample of the invention was smaller than the conventional example byapproximately 2.0 tonf.

At the next step, fluctuations in the forming load applied at the bottomdead center were evaluated by changing the dimensions of the concaveshape forming portion 3 b of the punch 3, thereby changing the ratio ofthe projected area A1 of the low Young's modulus member installationregion 3 d including the area directly under the concave shape formingportion 3 b to the projected area A2 of the concave shape formingportion 3 b. The results are indicated in FIG. 11.

As is obvious from FIG. 11, when the ratio of the projected area A1 ofthe low Young's modulus member installation region 3 d to the projectedarea A2 of the concave shape forming portion 3 b is 110% or greater,which is the scope of the present invention, the forming load isdrastically decreased. When the low Young's modulus member installationregion 3 d was shifted out of the area directly under the concave shapeforming portion 3 b, the forming load was similar with the resultsobtained when the ratio of the projected area A1 to the projected areaA2 in FIG. 11 was smaller than 100%. The forming load therefore couldnot effectively get reduced.

Although the examples are adopted to a press forming product having aconcave portion concaved in the press forming direction in the punchbottom, the effect of reducing the forming load around the bottom deadcenter is further exerted to such a press forming product that has aconvex portion convexed in the press forming direction in the punchbottom.

As has been proved, the press forming tool 1 according to the presentinvention allows a reduction in the forming load applied around thebottom dead center, by flexing the punch bottom forming surface 3 a ofthe punch 3 toward the low Young's modulus member installation region 3d formed under the punch bottom forming surface 3 a during press formingof the press forming product 11 having the concave portion 15 or convexportion concaved or convexed in the punch bottom 13 in the press formingdirection.

INDUSTRIAL APPLICABILITY

According to the present invention, a tool of press forming is providedthat allows a reduction in the forming load applied in the press formingprocess to produce a press forming product having a concave portion or aconvex portion concaved or convexed in the press forming direction inthe punch bottom, without employing any complex mechanisms orstructures.

REFERENCE SIGNS LIST

1 PRESS FORMING TOOL

3 PUNCH

3 a PUNCH BOTTOM FORMING SURFACE

3 b CONCAVE SHAPE FORMING PORTION

3 c LOW YOUNG'S MODULUS MEMBER

3 d LOW YOUNG'S MODULUS MEMBER INSTALLATION REGION

5 DIE

5 a PUNCH BOTTOM FORMING SURFACE

5 b CONCAVE SHAPE FORMING PORTION

7 BLANK HOLDER

9 BLANK

11 PRESS FORMING PRODUCT

13 PUNCH BOTTOM

15 CONCAVE PORTION

21 PRESS FORMING TOOL (CONVENTIONAL STRUCTURE)

23 PUNCH

23 a PUNCH BOTTOM FORMING SURFACE

23 b CONCAVE SHAPE FORMING PORTION

31 PRESS FORMING TOOL

33 PUNCH

33 a PUNCH BOTTOM FORMING SURFACE

33 b CONCAVE SHAPE FORMING PORTION

33 c LOW YOUNG'S MODULUS MEMBER

33 d LOW YOUNG'S MODULUS MEMBER INSTALLATION REGION

1. A press forming tool comprising: a punch; and a die, wherein thepress forming tool is configured to press form a press forming productincluding a punch bottom and a shallow concave portion or a low convexportion in the punch bottom, the punch includes a punch bottom formingsurface to form the punch bottom, a concave shape forming portion toform the concave portion or a convex shape forming portion to form theconvex portion in the punch bottom forming surface, and a low Young'smodulus member installation region formed under the punch bottom formingsurface and accommodating a low Young's modulus member having a lowerYoung's modulus than that of the punch bottom forming surface, and thelow Young's modulus member installation region includes an area directlyunder the concave shape forming portion or the convex shape formingportion of the punch and has a projected area in a press formingdirection that is 110% or greater as large as a projected area of theconcave shape forming portion or the convex shape forming portion.